arXiv subjects

Jernej F. Kamenik

Publications and source records attributed to Jernej F. Kamenik.

At least 55 records · Page 3Linked to original sources

arXiv2016

Confronting lepton flavor universality violation in B decays with high-$p_T$ tau lepton searches at LHC

We confront the indications of lepton flavor universality (LFU) violation observed in semi-tauonic $B$ meson decays with new physics (NP) searches using high $p_T$ tau leptons at the LHC. Using effective field theory arguments we correlate possible non-standard contributions to semi-tauonic charged currents with the $τ^+ τ^-$ signature at high energy hadron colliders. Several representative standard model extensions put forward to explain the anomaly are examined in detail: (i) weak triplet of color-neutral vector resonances, (ii) second Higgs doublet and (iii) scalar or (iv) vector leptoquark. We find that, in general, $τ^+ τ^-$ searches pose a serious challenge to NP explanations of the LFU anomaly. Recasting existing 8 TeV and 13 TeV LHC analyses, stringent limits are set on all considered simplified models. Future projections of the $τ^+ τ^-$ constraints as well as caveats in interpreting them within more elaborate models are also discussed.

hep-ph↗

arXiv2016

Comments on the diphoton excess: critical reappraisal of effective field theory interpretations

We consider the diphoton excess observed by ATLAS and CMS using the most up-to-date data and estimate the preferred enhancement in the production rate between 8 TeV and 13 TeV. Within the framework of effective field theory (EFT), we then show that for both spin-0 and spin-2 Standard Model (SM) gauge-singlet resonances, two of the three processes S to ZZ, S to Z gamma, and S to WW must occur with a non-zero rate. Moreover, we demonstrate that these branching ratios are highly correlated in the EFT. Couplings of S to additional SM states may be constrained and differentiated by comparing the S production rates with and without the vector-boson fusion (VBF) cuts. We find that for a given VBF to inclusive production ratio there is maximum rate of S to gauge bosons, b b-bar, and lighter quark anti-quark pairs. Simultaneous measurements of the width and the VBF ratio may be able to point towards the existence of hidden decays.

hep-ph↗

arXiv2016

Back to 1974: The $\mathcal Q$-onium

We show that the 750 GeV di-photon excess could be explained by the ${\mathcal Q}-$onium system of a new QCD-like theory with fermions vectorial under the SM. Beside the spin-0 di-photon singlet this scenario predicts almost degenerate colored scalars and spin-1 resonances analogous to the $J/Ψ$ in QCD. All these states are within the reach of the LHC. An apparent large width can be explained as due to production of excited states with splitting $Δm\sim Γ$.

hep-ph↗

arXiv2016

The phenomenology of the di-photon excess and $h\toτμ$ within 2HDM

The diphoton excess around $m_S=750$ GeV observed at ATLAS and CMS can be interpreted as coming from $S=H$ and $A$, the neutral components of a second Higgs doublet. If so, then the consistency of the light Higgs decays with the Standard Model predictions provides upper bounds on the rates of $S\to VV, hZ, hh$ decays. On the other hand, if $h\toτμ$ decay is established, then a lower bound on the rate of $S\toτμ$ decay arises. Requiring that $Γ_S\lesssim45$ GeV gives both an upper and a lower bound on the rotation angle from the Higgs basis $(Φ_v,Φ_A)$ to the mass basis $(Φ_h,Φ_H)$. The charged scalar, with $m_{H^\pm}\simeq750$ GeV, is produced in association with a top quark, and can decay to $μ^\pmν$, $τ^\pmν$, $tb$ and $W^\pm h$

hep-ph↗

arXiv2016

Indirect Constraints on the Scalar Di-Photon Resonance at the LHC

Motivated by the tantalizing excesses recently reported in the di-photon invariant mass spectrum at the LHC, we scrutinize some implications of scalar di-photon resonances in high energy proton-proton collisions. In particular, indications of a large width impose several challenges for model building. We show how calculability and unitarity considerations severely limit possible perturbative realizations of such a signal and propose a simple criterion that can be adapted to any renormalizable model. Furthermore, we discuss correlations between a di-photon excess and precision observables, including the anomalous magnetic and electric dipole moments of quarks and leptons, neutral meson oscillations and radiative flavor changing neutral current mediated decays of heavy leptons and hadrons. We find that existing searches and measurements significantly constrain the possibilities for a scalar resonance decaying into final states involving Standard Model fermions. We propose future search strategies which could elucidate some remaining currently unconstrained decay channels and discuss possible correlations between the di-photon excess and several recently reported flavor anomalies, showing that the latter can be addressed in a new incarnation of a gauged $U(1)^\prime$ model, with the di-photon resonance being the physical remnant of the $U(1)^\prime$-breaking field.

hep-ph↗

arXiv2016

Digamma, what next?

If the 750 GeV resonance in the diphoton channel is confirmed, what are the measurements necessary to infer the properties of the new particle and understand its nature? We address this question in the framework of a single new scalar particle, called digamma ($\digamma$). We describe it by an effective field theory, which allows us to obtain general and model-independent results, and to identify the most useful observables, whose relevance will remain also in model-by-model analyses. We derive full expressions for the leading-order processes and compute rates for higher-order decays, digamma production in association with jets, gauge or Higgs bosons, and digamma pair production. We illustrate how measurements of these higher-order processes can be used to extract couplings, quantum numbers, and properties of the new particle.

hep-ph↗

arXiv2016

Di-photon portal to warped gravity

The diphoton excess around 750 GeV observed by ATLAS and CMS can be interpreted as coming from a massive spin-2 excitation. We explore this possibility in the context of warped five-dimensional models with the Standard Model (SM) fields propagating in the bulk of the extra dimension. The 750 GeV resonance is identified with the first Kaluza-Klein (KK) excitation of the five-dimensional graviton that is parametrically lighter than KK resonances of SM fields. Our set-up makes it possible to realize non-universal couplings of the spin-2 resonance to matter, and thus to explain non-observation of the 750 GeV resonance in leptonic channels. Phenomenological predictions of the model depend on the localization of fields in the extra dimension. If, as required by naturalness arguments, the zero modes of the Higgs and top fields are localized near the IR brane, one expects large branching fractions to ttbar, hh, WW, and ZZ final states. Decays to a Z boson and a photon can also be observable when the KK graviton couplings to the SM gauge fields are non-universal.

hep-ph↗

arXiv2016

Searching for new spin-0 resonances at LHCb

We study the phenomenology of light spin-0 particles and stress that they can be efficiently searched for at the LHCb experiment in the form of dimuon resonances. Given the large production cross sections in the forward rapidity region together with the efficient triggering and excellent mass resolution, it is argued that LHCb can provide unique sensitivity to such states. We illustrate our proposal using the recent measurement of Upsilon production by LHCb, emphasising the importance of mixing effects in the bottomonium resonance region. The implications for dimuon decays of spin-0 bottomonium states are also briefly discussed.

hep-ph↗

arXiv2016

What is the gamma gamma resonance at 750 GeV?

Run 2 LHC data show hints of a new resonance in the diphoton distribution at an invariant mass of 750 GeV. We analyse the data in terms of a new boson, extracting information on its properties and exploring theoretical interpretations. Scenarios covered include a narrow resonance and, as preliminary indications suggest, a wider resonance. If the width indications persist, the new particle is likely to belong to a strongly-interacting sector. We also show how compatibility between Run 1 and Run 2 data is improved by postulating the existence of an additional heavy particle, whose decays are possibly related to dark matter.

hep-ph↗

arXiv2015

New Physics Models Facing Lepton Flavor Violating Higgs Decays at the Percent Level

We speculate about the possible interpretations of the recently observed excess in the $h \to τμ$ decay. We derive a robust lower bound on the Higgs boson coupling strength to a tau and a muon, even in presence of the most general new physics affecting other Higgs properties. Then we reevaluate complementary indirect constraints coming from low energy observables as well as from theoretical considerations. In particular, the tentative signal should lead to $τ\to μγ$ at rates which could be observed at Belle II. In turn we show that, barring fine-tuned cancellations, the effect can only be accommodated within models with an extended scalar sector. These general conclusions are demonstrated using a number of explicit new physics models. Finally we show how, given the $h \to τμ$ signal, the current and future searches for $μ\to e γ$ and $μ\to e$ nuclear conversions unambiguously constrain the allowed rates for $h \to τe$.

hep-ph↗

arXiv2015

Accidentally safe extensions of the Standard Model

We discuss a class of weak-scale extensions of the Standard Model which is completely invisible to low-energy indirect probes. The typical signature of this scenario is the existence of new charged and/or colored states which are stable on the scale of high-energy particle detectors.

hep-ph↗

arXiv2015

On the origin of the D0 like-sign dimuon charge asymmetry

We reconsider the recent observation by the D0 experiment of a sizable like-sign dimuon charge asymmetry, highlighting that it could be affected by CP-violating New Physics contributions not only in Bd- and Bs-meson mixings, but also in semileptonic decays of b and c quarks producing muons. The D0, measurement could be reconciled with the Standard Model expectations for neutral-meson mixings, provided that the CP asymmetry in semileptonic b (c) decays reaches 0.3 % (1%). Such effects, which lie within the available (rather loose) experimental bounds, would be clear indications of New Physics and should be investigated experimentally.

hep-ph↗

arXiv2015

Accidental matter at the LHC

We classify weak-scale extensions of the Standard Model which automatically preserve its accidental and approximate symmetry structure at the renormalizable level and which are hence invisible to low-energy indirect probes. By requiring the consistency of the effective field theory up to scales of 10^15 GeV and after applying cosmological constraints, we arrive at a finite set of possibilities that we analyze in detail. One of the most striking signatures of this framework is the presence of new charged and/or colored states which can be efficiently produced in high-energy particle colliders and which are stable on the scale of detectors.

hep-ph↗

arXiv2015

Dark Matter and Gauged Flavor Symmetries

We investigate the phenomenology of flavored dark matter (DM). DM stability is guaranteed by an accidental ${\mathcal Z}_3$ symmetry, a subgroup of the standard model (SM) flavor group that is not broken by the SM Yukawa interactions. We consider an explicit realization where the quark part of the SM flavor group is fully gauged. If the dominant interactions between DM and visible sector are through flavor gauge bosons, as we show for Dirac fermion flavored DM, then the DM mass is bounded between roughly $0.5$ TeV and $5$ TeV if the DM multiplet mass is split only radiatively. In general, however, no such relation exists. We demonstrate this using scalar flavored DM where the main interaction with the SM is through the Higgs portal. For both cases we derive constraints from flavor, cosmology, direct and indirect DM detection, and collider searches.

hep-ph↗

arXiv2014

Disentangling Flavor Violation in the Top-Higgs Sector at the LHC

We study the LHC phenomenology of flavor changing Yukawa couplings between top quark, Higgs boson, and either an up or charm quark. Such $tuh$ or $tch$ couplings arise for instance in models in which Higgs sector is extended by the existence of additional Higgs bosons or by higher dimensional operators. We emphasize the importance of anomalous single top plus Higgs production in these scenarios, in addition to the more widely studied $t \to h j$ decays. By recasting existing CMS searches in multilepton and diphoton plus lepton final states, we show that bounds on $tuh$ couplings are improved by a factor of 1.5 when single top plus Higgs production is accounted for. We also recast the CMS search for vector boson plus Higgs production into new, competitive constraints on $tuh$ and $tch$ couplings, setting the limits of $BR(t\to hu) < 0.7%$ and $BR(t\to hc) <1.2%$. We then investigate the sensitivity of future searches in multilepton channel and in fully hadronic channel. In multilepton searches, studying the lepton rapidity distributions and charge assignments can be used to discriminate between $tuh$ couplings, for which anomalous single top production is relevant, and $tch$ couplings, for which it is suppressed by the parton distribution function of the charm quark. An analysis of fully hadronic $t+h$ production and $t\to h j$ decay can be competitive with the multilepton search at 100 fb$^{-1}$ of 13 TeV data if jet substructure techniques are employed to reconstruct boosted top quarks and Higgs bosons. To show this we develop a modified version of the HEPTopTagger algorithm, optimized for tagging $t \to h j$ decays. Our sensitivity estimates on $BR(t\to hu)$ ($BR(t\to hc)$) at 100 fb$^{-1}$ of 13 TeV data for multilepton searches, vector boson plus Higgs search and fully hadronic search are $0.22%$ ($0.33%$), $0.15%$ ($0.19%$) and $0.36%$ ($0.48%$), respectively.

hep-ph↗

arXiv2014

Lepton flavor violation in type I + III seesaw

In the presence of a low scale seesaw of type I + III, flavor violating effects in the leptonic sector are expected. Their presence in the charged sector is due to the mixing of the fermionic vector-like weak triplets with the chiral doublets, which cause non-universality of the tree-level Z coupling. We investigate the bounds on the Yukawa couplings which are responsible for the mixing and present the results for two minimal cases, a fermionic triplet with a singlet or two fermionic triplets. Different channels for these processes are considered and their current and future potential to probe these couplings is discussed.

hep-ph↗

arXiv2013

Constraining Higgs mediated dark matter interactions

We perform an analysis of Higgs portal models of dark matter (DM), where DM is light enough to contribute to invisible Higgs decays. Using effective field theory we show that DM can be a thermal relic only if there are additional light particles present with masses below a few 100 GeV. We give three concrete examples of viable Higgs portal models of light DM: (i) the SM extended by DM scalar along with an electroweak triplet and a singlet, (ii) a Two Higgs Doublet Model of type II with additional scalar DM, (iii) SM with DM and an extra scalar singlet that is lighter than DM. In all three examples the Br(h to invisible) constraint is not too restrictive, because it is governed by different parameters than the relic abundance. Additional light particles can have implications for flavor violation and collider searches.

hep-ph↗

arXiv2013

Leptonic Monotops at LHC

We study the possibility of detecting New Physics (NP) phenomena at the LHC through a new search strategy looking at the monotop (top plus missing energy) signature which is common to a variety of NP models. We focus on the leptonic top decay mode and study the discovery or exclusion reach of the 2012 LHC data for three example models. Contrary to the hadronic mode, in this case the problematic QCD multijet background can be safely neglected. We find that the key kinematic variable to suppress most of the remaining SM backgrounds is the transverse mass of the charged lepton and missing energy. In fact, one could expect that the single-top production measurements already address the monotop signature in this mode. This is however not the case because in the SM single-top production the transverse mass has an end point determined by the W mass, while the NP signals typically have an additional source of missing energy. We compare, under the same conditions, our monotop search strategy with existing single-top measurements and find a considerable improvement in the monotop signature reach.

hep-ph↗